Our defence is not just the defenders. Macca runs his legs off closing down early, Palmer tracks(ed) back closing stuff down etc. When this doesn't work so well the back 3 come under more pressure. We see this with Palmer missing. I don't think we have anyone that plays the same way which shields the back line. That said, I think we lack a 'voice' at the back. None of our defensive unit options is a commanding character, I know it's noisy these days, but one of my memories form my early days at the Lane was the amount of shouting between players that went on. It seems very quiet these days in comparison. You could always hear Stubbs, Kilkline, McVay, Avromovic, Brown, McManus, Cherry, Short or Yates shouting to each other... or maybe they did in my imagination, but that was because they were big characters. Turner was a good organiser too. Someone needs to take that responsibility on, be it Slocombe or Stone or one of the CBs. We make poor decisions, especially when under pressure, so maybe one quick fix is to give one of the defensive unit that explicit responsibility - organise and manage the others and the defensive midfielders. Maybe that should be a key characteristic of any new defender we look at. It just strikes me that there is no clarity in what is expected and when. As a result our players try to play the ball at all costs even when their instinct is to get rid, resulting in sloppy passes, losing the ball in easy challenges etc.
One other thing I think is missing is being able to quickly transition from attack to defence, Sometimes it seems to take a few moments to re-orientate to set up or cover the defence. We got away with it last year, but at this level, that gets punished, and, as Shrewsbury showed, at the next level it will get punished ruthlessly.
Anyway I'm rambling...